A shock absorber is utilized to suppress vibration by being mounted in a building, a vehicle or the like. A damping valve is provided on a piston part of the shock absorber.
JP2005-48912A discloses a damping valve for a shock absorber 100. As shown in FIG. 5A, a damping valve includes a piston 101 configured to partition the interior of a cylinder S of the shock absorber 100 into an expansion-side chamber R1 and a compression-side chamber R2 filled with working fluid, a flow passage 103c formed in the piston 101 to allow communication between the expansion-side chamber R1 and the compression-side chamber R2 and a damping force generating element V laminated on a compression-side chamber side (lower side in FIG. 5A) of the piston 101 and configured to apply resistance to the working fluid moving from the expansion-side chamber R1 to the compression-side chamber R2 through the flow passage 103c. 
The damping force generating element V includes a plurality of leaf valves including a choke-forming leaf valve 104c composed of first to third leaf valves 140, 141 and 142 in the form of annular plates. The first, second and third leaf valves 140, 141 and 142 are arranged in this order from a piston side.
As shown in FIG. 5B, the first leaf valve 140 includes an outer peripheral part 140a which is seated on and separated from a valve seat (not shown) of the piston 101 and through holes 140b arcuately formed along a circumferential direction at an inner side of the outer peripheral part 140a. As shown in FIG. 5C, the second leaf valve 141 includes through holes 141a arcuately formed along a circumferential direction and cuts 141b formed from the arcuate through holes 141a to an outer peripheral end. As shown in FIG. 5D, the third leaf valve 142 is in the form of a circular plate including no through hole and no cut. The through holes 140b of the first leaf valve 140 and the through holes 141a of the second leaf valve 141 are arranged to vertically overlap (FIG. 5A).
In the case of laminating the first to third leaf valves 140, 141 and 142, upper and lower openings of the cuts 141b in FIG. 5A are closed by the outer peripheral part 140a of the first leaf valve 140 and the third leaf valve 142. Further, lower openings of the through holes 141a in FIG. 5A are closed by the third leaf valve 142. In this way, the through holes 140b of the first leaf valve 140 and the through holes 141a and the cuts 141b of the second leaf valve 141 constitute a passage which allows communication between the flow passage 103c and the compression-side chamber R2, and this passage can be caused to function as a choke.
When a piston speed is in a low speed region, the outer peripheral part 140a of the first leaf valve 140 is not separated from the valve seat of the piston 101. Thus, the shock absorber 100 can generate a damping force with choke characteristics due to resistance when the working fluid passes through the passage constituted by the through holes 140b, the through holes 141a and the cuts 141b. A damping characteristic (a change of the damping force with respect to the piston speed) in this case is a proportional characteristic as indicated by a solid line f1 of FIG. 6.
When a piston speed is in a medium-high speed region, an outer peripheral part of the leaf valve 104c constituting the damping force generating element V is deflected toward a side opposite to the piston 101 and the outer peripheral part 140a of the first leaf valve 140 is separated from the valve seat of the piston 101. In this way, the shock absorber 100 generates a damping force with valve characteristics due to resistance when the working fluid passes between the first leaf valve 140 and the valve seat. A damping characteristic (a change of the damping force with respect to the piston speed) in this case is a proportional characteristic as indicated by a solid line f2 of FIG. 6.
A shock absorber including through holes, which function as orifices, in a valve seat and leaf valves generates a damping force with orifice characteristics due to resistance when working fluid passes through the through holes in the case where a piston speed is in a low speed region. A damping characteristic (a change of the damping force with respect to the piston speed) in this case is a square-law characteristic as indicated by a broken line f3 of FIG. 6. Thus, in such a shock absorber, a damping coefficient (ratio of a damping force change amount to a piston speed change amount) is small and the damping force may be possibly insufficient when the piston speed is in a predetermined range (hereinafter, referred to as a “very low speed region”) from 0.
Contrary to this, in the shock absorber 100 including the passage shown in FIG. 5 which functioning as a choke, the damping characteristic when the piston speed is in the low speed region is the proportional characteristic as indicated by f1 of FIG. 6. Thus, the shortage of the damping force in the very low speed region can be suppressed.
Further, JP2008-138696A discloses a damping valve employing a divided piston structure. The damping valve includes a piston configured to partition the interior of a cylinder of a shock absorber into one chamber and another chamber filled with working fluid, a retainer (separator) laminated on another chamber side of the piston, a flow passage penetrating from the piston to the retainer and having an entrance constantly communicating with the one chamber, a leaf valve (expansion-side valve disc) in the form of an annular plate laminated on a side of the retainer opposite to the piston and configured to openably close an exit of the flow passage, and a piston rod penetrating through axial center holes of the piston, the retainer and the leaf valve.
The damping valve generates a damping force with valve characteristics similarly to the shock absorber 100 disclosed in JP2005-48912A. Further, in this damping valve, even if the flow passage passes on an inner peripheral side of the piston, the annular valve seat, on and from which the leaf valve is seated and separated, can be formed on the retainer to have a larger diameter. Thus, the leaf valve can be made larger in diameter and more easily deflected. Therefore, a damping coefficient (ratio of a damping force change amount to a piston speed change amount) when the piston speed is in the medium-high speed region can be made smaller.